The technology of coating substrates with a coating material has advanced in the fields of automotive parts, domestic electric appliances, building materials and the like for the purpose of rust prevention, coloration, brightening and so forth. In such coating, the substrate surface is generally subjected to the steps of under coating, intermediate coating, top coating, and the like to give a finished product. Among coating materials used for these coating steps, powder coatings in which no organic solvent is used are free of the risk of environmental pollution and give a good coat film appearance within a short curing time and, accordingly, they have been widely used in recent years.
Such powder coatings are produced, for instance, by admixing a thermosetting resin serving as a binder resin with a curing agent, a pigment and other additives, melting and kneading the mixture under conditions such that the crosslinking of resin molecules will not substantially proceed, extruding the same in the form of pellets and pulverizing the latter.
The thus-produced powder coatings are applied to substrates by such means as electrostatic spray coating or fluidized dipping method, followed by a baking step to give coat films.
However, the conventional powder coatings tend to readily undergo partial curing during the period between powder coating production and application and thus few are fully satisfactory from the storage stability viewpoint. Furthermore, only a small proportion of finished coat films have satisfactory coat film characteristics, in particular smoothness, luster, gloss, and the like.
In view of the foregoing, it is an object of the present invention to provide a powder coating excellent in storage stability and in coat surface smoothness.